<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script>
                // 0 1 2 3 4 5 6 7  
        var arr = [3,2,4,7,8,5,4,5]
        // var arr = [[3],[2],[4]]
        console.log('方法前原数组==>',arr)
        // console.log(arr.lastIndexOf(4,5))
        var res = arr.forEach((item,index,self)=>{
            console.log(item,index,self)
            // item[0]=5
            // item=5
            return 213212
        })
        console.log('返回值==>',res)
        console.log('方法后原数组==>',arr)

    </script>
</body>
</html>